PRETREATMENT

OPTIMIZATION

– A Key to Excellent Water Chemistry

By D K Bhargava AGM (Chem) Sandipt Varshney DGM (Chem) Rakesh Tatawat Chemist

WATER REQUIREMENTS IN POWER

PLANTS

CONDENSOR COOLING

AUXILLARY COOLING

FIRE FIGHTING

WATER TREATMENT PLANT

CLARIFIED WATER

DRINKING WATER

DM WATER FOR STEAM GENERATION

SOURCES OF RAW

WATER

SEA/OCEAN

RIVER

CANAL

PONDS

BOREWELL

The properties of water vary depending upon

its source.

Impurities Present

contain variable amounts of Suspended impurities Dissolved solids Organic impurities; These may be due to natural origin or due to

human activities (agriculture, industry, sewage), and these may vary with seasons, thus intake water quality will vary with change of seasons.

Problems due to Organics if not properly removed

in PT process

Some of the problems are: - Fouling of resin in DM plant leading to poor quality of

DM water. Extended rinse time for Anion during regeneration

resulting in lower output. High COD and TOC values of DM water High TOC and COD result in

Formation of organic acids in boiler water High ACC values of feed water & boiler water Boiler Tube Failures

1. 6 Feb 14, 2013

Due to long periods of water storage in reservoir and putrefaction of organic matter, aquatic plants ,leaves and branches of trees that fall in the reservoirs leads to deterioration of water quality. Thus the water received in PT plant contains low oxygen concentration & high load of organic, (COD & TOC values) & colloidal Silica.

Anta receives water from the RMC of river Chambal. Our station has three reservoirs of 20 lac M3 capacity. Reservoir water is enough for 90 to 100 days of power plant operations. Water treatment plant receives water from reservoir through Raw water pump house as per requirement.

Anta Experience

Affect

Taste

Color

Foul

Odor

(Smell)

Formation

of CH4 &

CO gas

Water

+

Organic

decaying

Matter

+

Micro organisms

Sump Pit (Closed)

Anaerobic Rxn.

Incomplete Decomposition

Formation of

Felvic acid

Humic Acids

Amines

Other organic Intermediates

Sludge

R E S U L T

Increase

TOC &

COD

values

Anaerobic Rxn.

1. 7 Feb 14, 2013

Process Raw water received from reservoir contains number of impurities. The impurities can be broadly divided into three categories;

1. Suspended clay / silt, colloidal silica. 2. Organic matter:- microorganisms, vegetation matter, organic matter & their

decay products such as Humus & Fulvic acid. etc. 3. Dissolved impurities First and Second type of impurities are removed in Pretreatment plant and the

dissolved impurities are removed in DM plant during formation of DM water. In the conventional pretreatment process Chlorine, Alum and Lime are mixed with

raw water in different quantities as per the impurities present in the raw water. • Chlorine is added to remove any microorganisms, organic acids & vegetable

matter etc. • Alum is added to remove suspended impurities like clay including colloidal

silica along with organic impurities, • Lime is added to control the pH of the clarified water

Studies Done

Jar test were done to determine the optimum alum, lime dozes and settling time.

Chlorine demand, organic content, DO, BOD, COD, TOC

values in raw water and clarified/ filter water were determined under different operating conditions of clarifier during different seasons.

The chlorine dozing was regulated to achieve break point

chlorine value of 0.5 ppm at clarifier outlet. Dissolved oxygen values were found to be low in summer

season when the raw water quality deteriorated.

Changes in Operation Philosophy

Both clarifiers were put into operation in January 2009 increasing the contact period in clarifier during

pretreatment and slightly higher Alum concentration then required to settle suspended impurities also helps in removing colloidal Silica and reducing organic loads during clarification process.

lime addition during post clarification also helped in improvement in Filtered water quality.

Storage Tank

Pressure Filter

Raw Water from Reservoir

Chlorine Lime Alum Clarifier 1

PROCESS FLOW SHEET OF PRETREATMENT PLANT

To DMplant

Raw Water

Clarifier 2

Retention Period with one clarifier in operation is around 3-3.5 hrs, where as in case of two clarifier operation it is around 6-7 hrs.

Changes in Operation Philosophy

Aeration in Raw water sump tank was started in March 2011 Aerating the raw water before pretreatment helps

in increasing the oxygen concentration in water, which helps in reducing the organic acids in water and bring down COD values (reduction in organic content).

No formation of sludge in raw water sump tank.

02/14/13 13

To WTP

Aeration in raw water

sump tank

Service Air Line

From

Reservoir

1. 14 Feb 14, 2013

After Aeration

Improved

Taste

No Color

No Foul

Odor

(Smell)

No Sludge

Formation

Water

+

Organic

decaying

Matter

+

Micro organisms

Sump Pit (Closed)

Aerobic Rxn. Presence of O2

Complete Decomposition

Formation of

Carbon dioxide

And

Water

R E S U L T

Decrease

in TOC &

COD

values

Air pipe

RESULTS

Results of Two clarifier operations Lower COD values of filtered water Decrease Colloidal silica. Less fouling of Anion resin Better output from DM plant. Achieved Mixed bed outlet K, as low as 0.08 µS/cm in summers. ACC (After Cation Conductivity) values of feed waters and steam

samples came down below 0.20 µS/ cm. low pH & high ACC problem frequently seen previously , disappeared.

Results of Aeration in Raw water sump pit. Low organics (COD) in feed water in PT plant, thus improving the

quality of filtered water further w r t organics. Foul smell, discoloration of raw water not seen. No Sludge formation in Raw water sump pit.

1. 17 Feb 14, 2013

Effect on ACC values

1. 18 Feb 14, 2013

Before Aeration After Aeration

PPM

Conclusion

Optimizing pretreatment would not only help in producing good quality clarified water, drinking water and DM water, but would also help in maintaining better feed cycle & boiler chemistry. Various hydro-chemical parameters like COD, BOD, organics, colloidal Silica, dissolved solids and suspended solids are to be monitored regularly during pre treatment process. Study shows that once the above parameters are controlled / removed during pre treatment then it is easier to maintain excellent feed cycle and boiler water chemistry in all respects.

JALDHARA QUALITY

CIRCLE

1. 20 Feb 14, 2013 Plant a Tree To get Oxygen Free

Month Jan-08 Feb-08 Mar-08 Apr-08 May-08 Jun-08 Jul-08 Aug-08 Sep-08 Oct-08 Nov-08 Dec-08

Raw water COD 20.0 – 26.0

18.0-24.0 15.8-25 15.4-18 11-31 10-25 10-22 15-24 15-20

Filter water COD 10-15 6.4-12 3.0-14 5.0-8.0 5.0-12.0 6.0-15.0 4.0-11.0 6.5-11 5.6-14.4

Mixed Bed O/L sp. Cond. 0.13-0.2 0.1-0.2 0.11-0.2 0.10-0.20 0.12-0.18 0.14-0.23 0.08-0.29 0.09-0.17 0.11-0.2 0.12-0.2 0.12-0.2 0.1-0.2

LP Boiler ACC 15-25 15-25 15-25 10.20 15-25 15-25 15-25 15-25 15-25 15-25 15-25 15-25

HP Boiler ACC 15-20 10-15 10-15 7-15 12-15 10-20 10-15 4-15 4-15 4-15 4-16 4-17

Live Steam ACC 0.152 0.175 0.195 0.175 0.180 0.175 0.180 0.142 0.130 0.132 0.132 0.132

Hotwell ACC 0.146 0.170 0.195 0.145 0.120 0.180 0.160 0.120 0.115 0.125 0.125 0.125

HP Feed Wtr ACC 0.145 0.178 .185 0.150 0.140 0.185 0.152 0.12 0.125 0.126 0.126 0.126

DM make up sp.cond 0.65-0.86 0.56-0.85 0.6-0.8 0.46-0.56 0.49-0.69 0.61-0.85 0.46-1.19 0.49-0.64 0.6-.095 0.6-0.785 0.6-0.85 0.6-0.75

Year 2008 Data Table

Month Jan-09 Feb-09 Mar-09 Apr-09 May-09 Jun-09 Jul-09 Aug-09 Sep-09 Oct-09 Nov-09 Dec-09

Raw water COD 18-25 12-34 18-32 15-21 18-22 17.6-25 15-27 15.4-26.4 11.3-32 12.6-16 15-20 18-22

Filter water COD 12-13 8-11 10-17 6.3-10 8-12 5-12 3-12 8-12 5-14 8-10 8-10 4-10

Mixed Bed O/L sp. Cond. 0.08-0.14 0.08-0.14 0.09-0.14 0.09-0.15 0.10-0.2 0.10-0.15 0.10-0.13 0.07-0.12 0.08-0.11 0.09-0.12 0.09-0.1 0.08-0.1

LP Boiler ACC 10-15 10-15 40-60 10-25 10-20 20-30 20-25 10-14 10-14 10-15 8-15 14-20

HP Boiler ACC 9-13 6-12 40-60 8-15 8-15 15-35 30-40 4-10 4-10 4-11 8-12 5-15

Live Steam ACC 0.16 0.11 0.125 0.150 0.170 0.2 0.17 0.13 0.09 0.1 0.168 0.18

Hotwell ACC 0.08 0.095 0.095 0.105 0.115 0.17 0.125 0.08 0.08 0.08 0.14 0.17

HP Feed Wtr ACC 0.1 0.100 0.130 0.127 0.140 0.12 0.140 0.138 0.095 0.10 0.184 0.17

DM make up sp.cond 0.4-0.6 0.42-0.6 0.45-0.6 0.42-0.65 0.41-0.65 0.5-0.65 0.49-0.6 0.37-0.55 0.45-0.6 0.48-0.6 0.53-0.6 0.5-0.6

Year 2009 Data Table

Month Jan-10 Feb-10 Mar-10 Apr-10 May-10 Jun-10 Jul-10 Aug-10 Sep-10 Oct-10 Nov-10 Dec-10

Raw water COD 14-24 14-20 18-27 18-36 30-42 30.5-46 20-30 27-29 24-35 10-16 12-20 12-20

Filter water COD 6-9 4-9 7-10 8-20 10-15 8-26 5-12 7-14 10-14 6-9 6-10 6-11

Mixed Bed O/L sp.

Cond. 0.08-

0.10 0.07-0.1 0.1-0.13 0.14-0.2

0.13-

0.16 0.12-

0.15 0.12-

0.15 0.13-

0.16 0.12-

0.15 0.11-

0.14 0.10-

0.15 0.09-

0.13

LP Boiler ACC 10-15 10-15 10-15 8-13 10.15 10-20 15-20 8-20 10-14 9-16 9-14 8-13

HP Boiler ACC 8-15 6-12 3-7 3-8 4-8 10-15 15-20 7-15 8-14 6-11 9-13 7-12

Live Steam ACC 0.12 0.11 0.125 0.120 0.085 0.14 0.13 0.1 0.11 0.09 0.1 0.08

Hotwell ACC 0.08 0.095 0.095 0.110 0.09 0.13 0.086 0.08 0.11 0.08 0.1 0.09

HP Feed Wtr ACC 0.1 0.100 0.130 0.127 0.08 0.14 0.16 0.11 0.14 0.1 0.12 0.11

DM make up sp.cond 0.5-0.6 0.5-0.6 0.54-

0.65 0.7-0.85 0.58-0.7

0.65-

0.75

0.58-

0.78 0.65-.8 0.7-0.8 0.65-0.7 0.6-0.7 0.6-.075

Year 2010 Data Table